Method and device for reading images of invisible radiation



Feb. 19, 1957 E. E. sHl-:LDON 2,782,332

METHOD AND DEVICE FDR READING IMAGES oE INVISIBLE RADIATION Filed April e, 1949 2 sums-sheet 1 IN VEN TOR.

21444190 2u/van. Saa oo/v NIIJ Feb. 19, 1957 E. E. sHELDoN METHOD AND DEVICE FOR READING IMAGES OF INVISIBLE RADIATION Filed April 6 1949 2 Sheets-Sheet 2 AI/M Wn/ IN VEN TOR. Emme@ MA/vlaez $442 WMV/Wm ,4 frcs/vs y nited States Patent METHOD AND DEVICE FOR READING IMAGES l F INVISIBLE RADIATION Edward Emanuel Sheldon, New York, N. Y. Application April 6, 1949, Serial No. 85,752

Claims. (Cl. 313-67) This invention relates to a method and device for storing and reading images of invisible radiations, and refers more particularly to a method and device 'for storing X-ray images, which term is meant to include other invisible radiations, such as gamma rays and the like, and also for images formed by irradiation by beams of atom particles, such as electrons or neutrons.

The primary purpose of this invention is to provide the possibility of storing the invisible images and inspecting them for a desired length of time when Wanted, without further expenditure of invisible radiation.

Another objective of this invention is to provide a method and device to produce intensified images for examination. This intensification will make it possible to overcome the ineiciency of the present uoroscopic examination. At present, illumination of the X-ray fluoroscopic image is of the order of (LODI-0.01 millilambert. At this level the human eye has to rely exclusively on scotopic (dark adaptation) vision which is characterized by tremendous loss of normal visual acuity in reference both to the detail and contrast.

Another objective of this invention is to make it possible to prolong the tluoroscopic examination, since it will reduce markedly the strength of radiation aecting the patients body. Conversely, the exposure time or energy necessary for examination using an invisible radiation may be considerably reduced. This will be of a great value in the military use of infra red rays.

Anotherobject of this invention is to provide a method and device to produce sharper and more contrasting images of invisible radiations than it was possible heretofore.

The present intensifying devices concerned with re` production of X-ray images are completely unsatisfactory, because at low levels of luorescent illumination, such as we are dealing with, there is not enough of X-ray photons to be absorbed by fluorescent or photoelectric screens used in such devices. Therefore the original X-ray image can be reproduced by them only with a considerablel loss of information. It is well known that the lack of sulicient number of X-ray quanta cannot be remedied by the increase of intensity of X-ray radiations, as'it will result in damage to the patients body. This basic deficiency of the X-ray examination was overcome in my invention by using an X-ray exposure of a strong intensity but of a short duration, and storing the invisible X-ray image for subsequent inspection for the desired length of time without any need of maintaining the `X-ray irradiation. The X-ray beam, therefore, can be shut oif while reading the stored X-ray image and in this way the total X-ray exposure received by the patient is not increased, in spite of using bursts of a great X-ray intensity. This storage of radar signals is well known in the art as evidenced by U. S. Patent No. 2,451,005 to P. K. Weimer, and other patents. The novelty of my invention consists of storing the images of invisible radiation and not only signals, and what yis even more important of storing simultaneously the total images instead of breaking them r"ce up into minute point images by scanning, in order to b e able to store them. Further deficiency of the present storage system is that the stored image can be reproduced with a good definition only if they are not intensified in the process of reproduction. ln my invention, intensitication of stored images is accomplished without sacricing detail and contrast of the stored image. This feature is ot' a great importance especially in X-ray examinations in which, without intensification of the order of 1000, the eye is confined to so-called scotopic vision at which it is not able to perceive definition and contrastV of the iiuoroescent X-ray image.

The purposes of my invention are accomplished by converting the invisible radiation images into photoelectron images by using the composite photocathode suitable for the particular kind of radiation applied, which photocathodes are described in detail in my co-pending application Serial No. 59,661 tiled NovemberY 5, 1948, now U. S. Patent No. 2,603,757, issued July l5, 1952. In case of dealing with X-ray radiation, said composite photocathode consists of light-reflecting layer, fluorescent layer, separating layer and photoemissive layer. ln case infra red radiation is used, such a composite'photocathode has infra red transmitting visible light reecting layer, such as gold, and infra red sensitive liuorescent layer, separating layer and photoemissive layer. Instead of composite photocathode in some instances a simple photocathode consisting of a layer emitting electrons under the influence of radiation applied and of a backing plate, may be used. The photoelectron image emitted from the photoemissive layer, and having the pattern of invisible radiation image is intensied by acceleration, demagnication and if necessary by sceondary emssion, and is focused by means of magneic or electrostatic lields on the image storage target consisting of a plurality of photoemissive islands separated by insulating areas and deposited on an insulating base. This photoelectron image produces in the storage target a charge image having the pattern of the original X-ray image. A strong broad'beam of infra-red or ultra-violet light is projected on the-storage target covering simultaneously all points of the target and causes a strong emission of the photo-electrons from the photoemissive islands of said target. The stored charge image on the target controls the emission of said broad photoelectron beam from said photoemissive islands. The emitted photoelectron beam is therefore modulated according to the pattern of the X-ray image and is focused on the viewing uorescent or other electron-reactive screen positioned in the same vacuum tube. The emitted photoelectron beam will reproduce the total image simultaneously and the reproduced image will be obviously greatly intensified as compared with the original image which wasone of the objectives of my invention. In view of the fact that the photoelectron image is deposited on the image storage target which is of dielectric or semi-conductor type, it will persist there for a long time. The storage time depends on resistivity of material used for the storage target and according to said resistivity may vary from a few seconds to a few minutes. During all this time it will be able to control the emission of strong broad photoelectron beam which produces the intensified nal image. In this way the image of invisible radiation can be stored and read when it is desired without any further expenditure or the invisible radiation, which is another purpose of this invention.

The invention will appear more clearly from the following detailed description when taken in connection with the accompanying drawings by way of example only preferred embodiments of the inventive idea.

In the drawings: Y Y

Fig. 1 represents a sectional view of the image storage tube;

Fig. 7 represents a plan view of a modified form of the composite target; and Y Fig. 7a represents la sectional view of a modified form of the composite target.

Reference will now be made to Fig. l illustrating the novel storage tube 1. The face of the image tube 9a must be of material transparent to the type of the depicting radiation to be used. Inside of the face of the tube there is a composite screen 13 comprising a very thin depicting radiation transparent and visible light reliecting layer 9 which prevents the loss of the light from the adjacent fluorescent layer 1G, very thin transparent layer 1,1 and photoemissive layer 12. The reflecting layer 9 may be of aluminum, gold or silver. The tiuorescent and photoemissive layer should be correlated so that under the iniiuenc'e of the depicting radiation there is obtainedI a maximum output of photoemission. More particularly the fluorescent layer should be composed of a material having its greatest sensitivity to the type of radiation to be used and the photoemissive material, likewise, should have its maximum sensitivity to the wavelength emitted `by the uorescent layer. Fluorescent substances to be used are zinc silicates, Zinc sulphides, zinc oxide, BaPtSOfx or organic phosphors, such as anthracene or napthalene with or without activators. The satisfactory photoemissive material will be in case of depicting radiation of infra red wave-length caesium oxide activated by silver. In case of depicting radiation of short wave-length such as X-rays or gamma rays, the best photoemissive surface would be of caesium, potassium or lithium on antimony or bismuth. The very thin transparent separating layer 11 may be of mica, silicates, quartz, or of a suit-able transparent plastic. The, invisible X-ray image of the examined body 14 is converted into a fluorescent image in the fluorescent layer `,and then into photoelectron image in the photoemissive layer 12. The photoelectron image is accelerated and focused. by the magnetic or electrostatic fields 13 and 18a.

on the composite screen 13a which produces the second photoelectron image. This second photoelectron image from the screen 13a is projected on va vstorage target 16 to' be described later. The focusing and accelerating fields are not indicated in detail as they are Well known in the art and their illustration will only serve to complicate the drawings. Sometimes it is better to demagnify the photoelectron image electronoptically before projecting it on said target. This can be' done by the use of electron lenses 19. The photoelectron image is focused on the target 16 with velocity causing secondary emission from the target at the ratio greater than unity (S greater than l). The secondary electrons emitted from the dielectric or semiconductive layer 4 of said target are drawn away by the adjacent mesh screen 26. In this way the photoelectron image is deposited as a positive charge image on the layer 4. The photoelectron image before being stored may be further intensified by the cascade amplification described in detail in my U. S. Patent No. 2,555,424 and U. S. Patenti No. 2,555,423, issued .lune 5, 1951. In this case, the photoelectron imagefrom the composite photocathode 13,- is focused on a similar composite screen 13a representing the second stage for further intensification and thereafter on the dielectric storage target 16.

It isrobvious that photoelectron image can be also focused on the storage target with velocity at which secondary electron emission is smaller than unity (S The resulting charge image will then be a negative one. In such a case, the mesh screen 20 may be abandoned. The storage target 16 consists of dielectric plate 4 such as of quartz, glass, or of a suitable plastic having metallic frame 2 around its borders connected to the outside source of potential. A fine metallic grid 5 is evaporated on this dielectric plate, so that each grid line comes in contact with the metallic frame 2. Next photo-emissive islands 3 are produced on said dielectric plate by evaporation through a fine mesh screen in such a manner that each photoemissive island is in contact with metallic grid line. The construction of such target is shown in detail in Fig. 6. The general idea of such targets is well known in the art as evidenced by the articles in the Air Technical Data Digest, vol. 12, No. l, July l, 1947, page ll. The storage target 16 may be Ialso constructed in such a manner that the insulating islands are inserted into the photoemissive base instead of arrangement shown in Fig. 6. The preferred way of construction of this type of storage target 16a is shown in Fig, 7 and Fig. 7a. In this modification minute holes 63, 25-4() microns in diameter, are drilled in the photoemissive layer 62. T he holes are filled with quartz or with la plastic of suitable dielectric properties by extrusion process, which results in creation of uniformly spaced identical in size and shape insulating islands 64 embedded in the photoemissive base. The photoemissive layer has a transparent conducting backing plate 65 such as of aluminum, magnesium or silver. The transparent backing plate 65 may be also used in the form of wide mesh metallic screen. This type of target represents in construction the opposite of the target 16, but the operation of both targets is the same. A strong source of invisible light, such as ultra-Violet or infra-red 6, is projected on said composite target. Under the influence of invisible light a strong broad beam of photoelectrons is emitted from the photoemissive islands 3. The photoemission is. controlled and modulated by the pattern of charges stored in the insulating islands 4a between the photoemissive islands 4. In particular the greater the positive charges stored in the insulating islands 4a are, the more the emission of photoelectrons will be suppressed. Therefore, the beam of photoelectrons which is emitted from the storage target 16 will have imprinted on it the pattern of the original invisible X-ray image. The emitted photoelectron beam 24 is of a much greater intensity than the original X-ray image, therefore, by converting said transmitted electron beam into a visible image in the iiuorescent screen 25, a marked intensification of the original X-ray image is obtained. ln this Way the stored X-ray image may be read for a desired length of time without any further expenditure of X-ray energy which was the primary objective of this invention. The fluorescent screen 25 has-an electron transparent, light reecting layer 25a such as of aluminum, to prevent destructive back-scattering of light. Instead of fluorescent screen, other electron reactive surfaces may be used such as photographic film, electrolytic papers or electrographic plates. The emitted photoelectron beam 24 before its reproduction into visible image may be intensified-by acceleration by fields 28 and 28a and by electron-optical demagnification. In some instances itfmay be necessary to use in addition, a secondary electron emissive electrode 26. such as of Css-Sb, CsOAg or of insulator, like mica or glass or of a metal such as Mg, Ni or Be, on whichphotoelectron beam 24 is focused for further intensification. The photoemissive islands 3 in the storage target may be` of caesium, potassium or lithium on antimony when using ultra-violet source of light, or of caesium or lithium onl bismuth or arsenic when using White light. In some instances, it ispreferable to use infra-red source of light because electronsernitted by infra-red are more uniform in their ramass velocities, which is of importance for resolution of the image. In such a case, the photoemissive layer should be preferably of caesium silver oxide. Dielectric layer 4 has to be of material capable of storing the electrical charge image for a long time. Suitable materials for such purpose are mica, quartz, glass or plastics of a great electrical resistivity. The walls of the image storage tube should be coated with a light non transparent material except for the area 29, through which ultraviolet light is projected on the storage target 16. This is necessary in order to prevent spurious signals from the photocathode 13 or 13a.

The action of accelerating and focusing fields 1S, 18a and 28 is of sequential character. At the time of the X-ray exposure, the fields 18 and 18a are simultaneously activated, whereas fields 28 remain inactive. At the time of reading the stored X-ray image, when ultra-violet source is in operation, the fields 28 and 23a are simultaneously activated, whereasfields 1S and 18a are inactive. Also the action of the mesh screen is intermittent. At the time of the X-ray exposure the mesh screen 20 is connected to the ground or to source of potential. At the time of the ultra-violet exposure the mesh screen 2l) is disconnected. Also the action of ultraviolet light is of intermittent character, being used only when the stored X-ray image is to be read. After the X-ray image has been read in the fluorescent screen 25, the composite target 16 has to be restored to the original condition before the next X-ray image can be stored. rlfhe dielectric layer 4 at the end of the reading has a positive or negative charge thereon according to the velocity of photoelectrons from the composite photocathode 13. In order to neutralize this charge, the composite photocathode is sprayed with uncontrolled X-ray beam from the X-ray tube 38 and the emitted photoelectrons are projected on the dielectric layer 4 with a velocity causing secondary electron emission therefrom of the sign neutralizing the remaining charges.

In some cases, in order to avoid electron-optical diiiiculties resulting from the oblique projection of the X-ray image onto storage target, the image storage tube may have the configuration shown in Fig. 2. In this modification of my invention, the composite invisible radiation sensitive photocathode 13 is placed at one end of the evacuated tube 3@ and the source of light 6 `at the opposite end of said tube so that photoelectron image and the light beam are projected normally on the storage target. In this case the dielectric layer 4 must be transparent to the light used. For ultra-violet, quartz will be a suitable material. The photoemissive islands 3 must be of translucent type which means that the photoelectrons are emitted from the side opposite to the light source. The original X-ray image converted into photoelectron image in the composite photocathode is accelerated and focused on the composite target by the action'of magnetic or electrostatic fields 34. The photoelectron` image emitted by irradiation with a strong source of light 6 is deflected by magnetic or electrostatic fields 33 which are not shown in detail as they are well known in the art, onto fluorescent or other electron reactive screen 32. The screen 32 has an electron transparent light reflecting backing layer 32a. The original X-ray image is reproduced therefore on said screen with a marked intensification. The mesh screen 20 is not essential for the operation o-f my invention and may be omitted When the best resolution of the image is necessary.

Further improvement in electron-optical geometry may be achieved by projecting the final electron image on the viewing 'screen normally instead yof in oblique projection. Thi-s modification is shown in Fig. 3. In this case, composite screen 13 converting the invisible X-ray image into photoelectron image is disposed at one end of the image storage tube 40, so that said photoelectron image can be focused normally on the storage target 42. The focusing and accelerating iields 41 'produce velocity of electrons at which they can pass through the thin dielectric storage target and produce a charge image on the opposite side thereof. The storage target 42 consists of a wide supporting mesh screen 43 on which a thin electron transparent continuous layer such as of aluminum or magnesium 44a is deposited, and on the latter a thin dielectric layer 44 such as of quartz or of suitable plastic is deposited. The dielectric layer 44 i-s similar in construction to the storage target 16 described above having a metallic frame 2a, a thin metallic grid 5a and photoemissive islands 3a deposited on its surface. The electron transparent layer 44a is not essential for the operation of this target and may be omitted. In such a case the supporting mesh screen should be of a conducting material, such as of metal. The charge image on insulating islands 4a between said photoemissive islands controls the photoemission from the photoemissive islands -3a caused by irradiating them with the light source 6, as described above. The emitted photoelectron beam is accelerated and if necessary, intensified by electron-optical demagnication and by secondary emission, in the special secondary electron emissive electrode 45. The latter consists of a wide mesh supporting screen 45a with a thin secondary electron emitting layer 45b such as of Mg, Ni, Be or of a suitable alloy deposited thereon. This arrangement allows the secondary electrons to emerge from the side of the electrode opposite to the source of the primary electron beam. The electron image multiplied by secondary emission is focused on the fluorescent or other electron reactive screen 25 and reproduces the original X-ray image.

In some cases, instead of having composite photocathode 13, a photocathode of photoemissive layer 46 on the X-ray transparent backing plate 47 is used, as shown in Fig. 4. In such a case, the X-ray image is projected on the uorescent screen 48 positioned outside of the image tube and the resulting fluorescent image 14a is projected by refractive 49 or reflective optical -system on said photocathode. The rest of the operation of the image storage system is th-e same as described above.

In other cases, instead of having a composite photocathode 13 or photocathode 50, a simple electron emissive photocathode of a thin layer of a heavy metal, such as lead, gold, bismuth or uranium mounted within the vacuum tube, may be used. The photoelectron image from said photocathode is focused on the storage target, as described above.

It is obvious that my invention may be used not only for X-ray images but as well for images produced by other invisible radiations whether of undulatory or of corpu'scular nature. In particular, my image storage tube may have application for the storage of infra-red images. In such a case, the composite photocathode 13 or` 50 has to be made sensitive to infra-red rays. Such a photocathode may be of photoemissive layer CsOAg on infra-red transparent backing plate. l

The invisible radiation sensitive photocathode 13 and Si) and the storage target 16 or 42 may be obviously disposed in many different ways in relation to each other. All these modifications are obviously Within the scope and spirit of my invention..

Instead of using light radiation to produce a broad beam of photoelectrons, a special electron gun delivering a broad beam of electrons may be provided. The electron gun may be used in all modifications of my invention described above.

Another improvement in the application of electron gun in my invention is shown in Fig. 5. The X-ray image is converted by the composite Screen 13 into a photoelectron image, the photoelectron image is accelerated and focused by fields on the storage target 51. The target 51 consists of electron transparent light refleeting, conducting layer 52, fluorescent layer' 5,3 and transparent dielectric layer 54 having deposited thereon multiple photoemissive islands 3. In somefcases it is advantageous .to use in addition a special light transparent conducting layer 52a, between the fluorescent layer 53 and dielectric layer 54. The light reflecting layer 52 may be of aluminum or gold. The fluorescent layer may be of zinc sulphides, selenides, BaPtSO4 or of ZnO. Dielectric layer 54 may be of quartz, glass or a suitable plastic. The photoemissive islands 3 may be of caesium, potassium or lithium on antimony or bismuth or of caesium silver oxide. The photoelectron beam 5S having the pattern of the original X-ray image is focused on the target 51, penetrates through the light reflecting layer 52 and is converted in the fluorescent layer 53 into a fluorescent image. The fluorescent image is transmitted through the transparent dielectric layer 54 and causes photoemission from the photoemissive islands 3. As a result the positive charge image is left on the photoemissive island 3 and can be stored for a long time because of dielectric properties of the layer 54 surrounding said islands. broad beam of electrons of high intensity 57. The electron beam 57 when impinging on the target 51 is causing secondary electron emission which is modulated by the pattern of electrical charges present thereon. The secondary electron beam 58 is accelerated by magnetic or electrostatic fields 6l and if necessary is intensified by electron-optical demagnification and secondary emission from the -secondary electron emissive electrode, as was described above. The intensied electron beam is focused on the fluorescent or other electron reactive screen 25 reproducing the original X-ray image.

Instead of using the target 51, the storage target 42 shown in Fig. 3 may be also applied with good results. In somecases, the mesh screen 59 positioned in a close proximity to the scanned side of the storage target and connected to the source of positive potential may be advantageous, as it contributes to increasing of the output of signals.

Instead of a continuous broad beam of electrons having the diameter of the storage target, a flat ribbon type of scanning electron beam may be also used, which covers simultaneously one line of the image. The flat ribbon type of scanning beam is well known in the art, and therefore 4does not have to be described in detail here. The broad electron beam should be preferably of the low velocity. In some applications, however, a high velocity electron beam may be used as well. It is obvious that the composite photocathode 13, the target 51 and the electron gun 56 may be disposed in many different ways.

The operation of this image storage tube is also of intcrmittent type as was described above. When the X-ray source 38 is operating, the fields 60 are activated and the electron gun 57 as well as fields 6i remain inactive then. When the X-ray image is to be read, the X-ray source 38 and the fields 60 are inactive, whereas the electron gun 57 and the fields 61 are operating now.

My invention can be also used for intensification of images of invisible radiations, without storing them prior to their reading. In such a case, the dielectric target has to have the conductivityallowing the stored image charge to dissipate completely within a desired length of time. In this case, operation of the image tube and of X-ray source has to be continuous instead of being intermittent, as was described above. It should be understood that the storage targets described above may be used in various vacuum tubes, such as'photo-multiplier tubes and pick-up tubes.

The sensitivity of invisible radiation image storage tube can be markedly increased by the use of a storage phosphor for the fluorescent layer 10 in the composite photocathode 13. The invisible X-ray image is stored in said phosphor and is released therefrom in the form of fluorescent image only after irradiation with an additional source of radiation, such as infra-red. In this case, the visible light reflecting layer `9 obviously must be transparent to The electron gun 56 is producing a i.

infra-red radiation. A thin layer of gold will be suitable for this purpose. Satisfactory phosphors for the storage of images are alkaline earth sulphides and selenides activated with cerium, samarium or europium, sulphidcs rctivated with lead or with copper, or lanthanum oxysulphides with activators. ln case image storage tube should serve for storage and detection of infra-red images, the photocathode should be irradiated with ultra-violet radiation from an extraneous source after the exposure to infra-red image. In such case, the light reflecting layer 9 obviously has to be transparent to ultra-vio'let light. The phosphor of the fluorescent layer may be the same as described above, that is, of alkaline earth sulphides and selenides activated with cerium, samarium or europium, sulphides activated with lead or with copper, or lanthanum oxysulphides with activators. The storage phosphors are more sensitive to electrons than to X-rays. A good way of taking advantage of greater sensitivity of storage phosphors to electrons is to use them in a fluorescent layer in a, composite screen 13a representing the second stage of the image section, see Fig. l. This arrangement is advantageous because in such case the storage phosphor is excited by the electron beam having the pattern of thc X-ray image instead of by the invisible X-ray image itself. lt should be understood that the word target in the specification and in the appended claims represents a screen.

It will thus be seen that there is provided a device in which the several objects of this invention are achieved and which is well adapted to meet the conditions of practical use.

As various possible embodiments might be made of the above invention, and as various changes might be made in the embodiments above set forth, it is to be understood that all matter herein set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A composite device comprising in combination an imperforated and continuous layer of material having dielectric and piezo-electric properties, said layer having an extended surface, a conducting member adjacent to one side of said dielectric layer and photo-electric means v mounted on the opposite side of said dielectric layer and comprising photo-emissive material.

2. A vacuum tube comprising in combination a composite device having a layer of material having dielectric and piezo-electric properties, a conducting member adjacent to one side of said dielectric layer and photoelectric means mounted on the opposite side of said dielectric layer and comprising photo-emissive material emitting electrons, and an imperforated screen facing said photo-electric means and receiving said emitted electrons, said screen comprising a light reflecting layer and a fluorescent layer.

3. A device as defined in claim l, which comprises in addition a second conducting member, said second member being in contact with said photoelectric means.

4. A device as defined in claim 2, which comprises in addition a second conducting member, said second member being in contact with said photoelectric means.

5. A vacuum tube comprising a composite device having in combination a rst conducting member, an imperforate layer of dielectric and light transparent material, said first conducting-member being mounted on one side of said dielectric layer and being light reflecting, fluorescent means in contact with said first conducting member, a second conducting member mounted on the opposite side of said dielectric layer, and photoelectric means in contact with said dielectric layer and comprising a plurality of photoelectric islands, said first conducting member forming a continuous surface which is parallel to the major surface of said fluorescent means.

6. A device as defined in claim 5, in which said first conducting member is transparent to electrons and in 9 which device said photoelectric means have an exposed surface.

7. A vacuum tube comprising within said tube in combination a composite device having a first conducting member, an imperforated layer of dielectric material, said first conducting member being mounted on one side of said dielectric layer in contact with said dielectric layer and being continuous,and a second conducting member mounted on the opposite side of said dielectric member, at least one of said conducting members being parallel to the major surface of said dielectric layer, photoelectric means adjacent to the major surface of said dielectric layer and in contact with one of said conducting members, having an exposed surface and comprising photoemissive material emitting electrons, and a screen spaced apart from said photoelectric means comprising a light-reflecting layer, and a fluorescent layer for receiving said electrons.

8. A device as defined in claim 7, in which said dielectric layer is light-transparent and which comprises in addition uorescent means.

9. A vacuum tube comprising Within said tube in cornbination a composite device having a first conducting member, an imperforated layer of dielectric material, said layer being discontinuous from Walls of said tube, said first conducting member being mounted on one side of said dielectric layer and being light-reflecting, fluorescent means, a second conducting member mounted on the opposite side of said dielectric layer, photoelectric means in contact with the major surface of said dielectric layer, having an exposed surface and comprising photo-emissive material emitting electrons, said first conducting member being substantially parallel to the major surface of said dielectric layer and of said photoelectric means, and a screen spaced apart from said photoelectric means comprising a light-reflecting layer and a fluorescent layer, and receiving said electrons from said photoelectric means.

10. A vacuum tube comprising in combination a composite device having a first conducting member, imperforated means of dielectric and light-transparent material, said first conducting member being mounted on one side of said dielectric means, and being of light-refleeting material, fluorescent means, a second conducting member mounted on the opposite side of said dielectric means, photoelectric means mounted in contact with said dielectric means, having an exposed and continuous surface and comprising photoemissive material emitting electrons, and a screen spaced apart from said photoelectric means comprising secondary electronemissive material, receiving said electrons from said photoelectric means and emitting a plurality of secondary electrons for each electron received.

11. A device as defined in claim 9, in which said photoelectric means comprise a compound of the group consisting of antimony` and bismuth.

l2. A vacuum tube comprising in combination a composite device having a first conducting member, imperforated means of dielectric and light-transparent material, said first conducting member being mounted on one side of said dielectric means and being of light-I reflecting material, a second conducting member being mounted on the opposite side of said dielectric means, uorescent means mounted on one side of said dielectric means, photoelectric means deposited in contact with the side of said dielectric means opposite to the side on which said fluorescent means are mounted, said photoelectric means comprising antimony and a compound of the group consisting of potassium and lithium, having an exposed surface and emitting electrons, and a screen spaced apart from said photoelectric means, comprising secondary electron-emissive material, receiving said electrons from said photoelectric means and emitting a plurality of secondary electrons for each electron received.

13. An invisible radiation storage tube comprising in combination a screen for receiving and converting an invisible radiation image into a first broad electron beam having the pattern of said image, a storage target comprising dielectric means for receiving and storing said electron beam on the surface of said storage target which is facing said image receiving and converting means and a conducting member in contact with said dielectric means, and photoelectric means for producing a broad photoelectron beam, the emission of said broad photoelectron beam being modulate-d by said stored electron beam.

14. An invisible radiation image storage tube comprising in combination means for receiving and converting an invisible radiation image into a rst electron beam having the pattern of said invisible image, a storage target comprising a dielectric layer for receiving and storing said electron beam on the surface of said storage target which is facing said image receiving and converting means, a plurality of photoemissive islands deposited on said dielectric layer for producing a second electron beam, said second electron beam being a broad beam, the emission of said second electron beam being modulated by said stored electron beam and an electronreactive imperforate screen comprising an imperforate electron pervious layer for receiving said modulated second electron beam.

15. A device as defined in claim 13, in which said means for converting an image of invisible radiation into an electron image comprise a composite screen having a fluorescent layer and a photoemissive layer.

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